The often explained reason why no one thinks Clark Kent is Superman is pretty straight forward. People think Superman is always Superman. They are unaware that he uses a secret identity to blend in. While other Comic Heroes like Batman wear a mask, begging the question, who is this guy behind the mask? Superman doesn't have a mask. He is Superman. There are probably a lot of dudes that sort of favor Superman. But no one is looking for Superman's secret identity because they don't know he has one. If Clark looks like Superman, so what. He ain't Superman.